A major challenge in breast cancer research is to identify accurate markers for early diagnosis, prediction of tumor aggressivity, and determination of the most appropriate course of treatment. One approach to finding such markers is to develop a molecular profile that can distinguish between different cancer types. This proposal will examine the potential of transfer RNAs (tRNAs) to serve as breast cancer biomarkers, as well as their role in regulating the development and progression of breast cancer cells. Transfer RNAs are best known for their role in translation, where they read mRNA codons and bring the appropriate amino acid to the nascent polypeptide chain. An unexpected finding from the human genome sequence is the large diversity of gene sequences coding for tRNAs, well beyond that needed for translation. This large diversity in sequence of human tRNAs suggests that cancer cells may generate unique patterns of tRNA expression, which could be useful as biomarkers for cancer types or progression. Unique tRNA microarray technology will be applied to profile tRNA expression levels in transformed and normal breast cell lines. Preliminary results indicate tRNA levels will change with oncogenic transformation, and may exhibit selective patterns based on upregulation of distinct signaling pathways. Establishing the potential of tRNAs as biomarkers will open up a new avenue for detecting the type and stage of breast tumors. In addition, overexpression of certain tRNAs may drive oncogenic transformation. Initiator methionine tRNA and other tRNAs strongly upregulated in breast cancer cell lines will be overexpressed in normal breast cells to determine whether they can drive cell proliferation and transformation. Validation of this mechanism will establish modulation of tRNA expression as a novel means for controlling the development and progression of cancer cells. Such tRNAs and the genes involved in their regulation could be targets for a new class of cancer drugs. Worldwide, breast cancer is the second most common type of cancer. A major challenge in breast cancer research is to identify accurate markers for early diagnosis, prediction of tumor aggressivity, and determination of the most appropriate course of treatment. This work will evaluate the potential of transfer RNAs to serve as biomarkers for breast cancer and their role in the development of breast cancer cells.